The hysteresis observed in cyclic acid-base titrations of the three-stranded polyribonucleotide helix poly (A) 2 poly(U) strongly depends on ionic strength. For NaCl and at 25° C, hysteresis occurs in the limited concentration range between 0.03 M and 1.0 M(NaCl). The transition points, associated with the cyclic conversions between the triple helix and the poly(A) poly(A) double helix and (free) poly(U) constitute a (pH ionic strength) phase diagram covering the ranges of stability and metastability of the hysteresis system. Variations with NaCl concentration of some hysteresis parameters can be quantitatively described in terms of polyelectrolyte theories based on the cylinder-cell model for rodlike polyions. The results of this analysis suggest that the metastability is predominantly due to electrostatic energy barriers preventing the equilibrium transition of the partially protonated triple helix above a critical pH value. Ultraviolet absorbance and potentiometric titration data of poly (A) in the acidic pH range can be analyzed in terms of two types of double-helical structures. Spectrophotometric titrations reveal isosbestic wavelengths for structural transitions of poly (A). Time effects commonly observed in poly (A) titrations are suggested to reflect helix-helix transitions between the two acidic structures.